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LIGO-G1000559-v1 2 A mystery in the heart of the Milky Way UCLA astronomy Andrea Ghez made infrared observations of the center of our Galaxy, the Milky Way. Here is a time lapse movie of her observations, showing star positions from 1995 until now. Note that the stars appear to be orbiting an unseen object. From the orbits, we can determine the mass of the unseen object. It is about 4 million solar masses! Is it a black hole?

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LIGO-G1000559-v1 3 What is a black hole? Imagine compressing all of the mass of the Sun into a sphere about four miles in diameter. If you could do that, what would it be like? The gravity at the surface would be so strong that nothing could escape, not even light. Once that happened, gravity is also so strong that no force can keep the star from collapsing completely under its own weight. It becomes a point (!), containing all of its original mass.

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LIGO-G1000559-v1 4 Are black holes a threat? Will black holes suck up everything in the Universe? No. Their strong gravity is only strong very close to the black hole. The event horizon marks the ghostly reminder of the surface of the star, at the size that just barely can’t let light escape from it. Further away from the star, gravity falls off, getting weaker just as it would for matter in a more ordinary form. If the Sun suddenly became a black hole, its gravity at Earth’s distance would be the same, and we’d orbit like before. (Of course, we’d miss the light!)

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LIGO-G1000559-v1 5 Black holes are interesting! Black holes consist of matter in one of its most extreme forms ever imagined. So dense, that it almost isn’t matter. All that is left of its character is its mass. Otherwise, to quote a famous explanation, we say that “A black hole has no hair.” (It has no individual features.) Another way of looking at a black hole is that it consists of pure gravity, or in Einstein’s terms, it consists of pure “space-time curvature”.

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LIGO-G1000559-v1 6 Einstein’s view of gravity: The General Theory of Relativity Starting in 1915, Albert Einstein began the development of a new theory of gravity. The basic idea is that gravity is not a force, but rather a manifestation of the curvature of space-time. Space and time aren’t just a simple backdrop to the world, but have properties of their own. In particular, they can be “curved”, which means that matter can be prevented by the properties of space-time from moving uniformly in a straight line. Space-time curvature is caused by mass. Thus, General Relativity embodies the idea of gravity, and even “explains” it.

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LIGO-G1000559-v1 8 Black holes, from the point of view of General Relativity A view of the space-time in the vicinity of a black hole. In the region where the escape velocity exceeds c, the geometry of the curved space- time becomes extreme.

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LIGO-G1000559-v1 9 The sad fate of matter that forms a black hole No force can hold up the matter that forms a black hole. All of the matter inside collapses down to a point.

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LIGO-G1000559-v1 10 Are they really out there? The idea of black holes is pretty exotic. We’d like to know if black holes actually exist. If they do, what are their properties? How massive? How many? At first, it seems unlikely that we could ever know. After all, if even light can’t escape from a black hole, how could be observe it? Nevertheless, evidence is accumulating that black holes do exist. Now, I’ll explain a new way of looking for black holes that will let us get “up close and personal” with them, so that we can study them in detail.

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LIGO-G1000559-v1 11 Black hole vibrations create space-time ripples If a black hole is disturbed, distorted, or newly-created, it will vibrate. A vibrating black hole launches ripples in space-time, also known as gravitational waves. If two pre-existing black holes collide, they will form a new larger (but momentarily distorted) black hole. This happens often, from black holes in binary pairs, two black holes orbiting each other. Gravitational waves are made by the orbiting black holes. This carries away energy, causing them to spiral towards each other, eventually colliding and forming a single more massive black hole.

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LIGO-G1000559-v1 13 The space-time ripple (red in simulation) as it passes us

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LIGO-G1000559-v1 14 With the right “microphone”, we could listen to these ripples Here’s a playback of the gravitational wave from the collision of two black holes, played back directly through loudspeakers. Other than amplification, no other changes necessary in order to make this audible. The ripples naturally occur in the (human) audio band!

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LIGO-G1000559-v1 15 How to make a “microphone” for space-time ripples

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LIGO-G1000559-v1 23 Where are we in the search for gravitational waves? We have been collecting observations and analyzing data with initial LIGO for several years. So far, no luck in finding gravitational waves. Later this year, we’ll disassemble our instruments to begin installation of Advanced LIGO, with 10 times the present sensitivity. By 2015, Advanced LIGO will be ready. It will have enough sensitivity to find gravitational wave signals. Then, we’ll be ready to explore the Universe using this new “ear” for space-time ripples, and look into the nature of black holes.

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